The present invention relates to passive radio-frequency circuitry and, more particularly, to a novel radio-frequency (RF) four-way power splitter/combiner apparatus operating in a push-pull mode.
In many high-power RF applications, such as magnetic resonance imaging and the like, vacuum tube amplifiers are utilized to provide the relatively high RF excitation energy needed. However, solid-state devices capable of generating RF power outputs of the required magnitude are available at lower RF frequencies, and are expected to be shortly available at RF frequencies in the lower VHF region; use of solid-state power amplifiers is highly desirable, to provide the desired excitation signal: with larger bandwidth; over a longer lifetime (higher reliability); require simpler cooling systems; and cost less. At present, providing even a high-frequency RF power amplifier with significant, e.g. greater than several hundred watts, power output requires that the output of several individual power amplifier modules, each capable of providing only a portion of the required output power, be combined. The input power for the plurality of amplifier modules must itself be provided by splitting a common input signal. Accordingly, it is highly desirable to provide apparatus having a common terminal and four other terminals, a first pair of which is at a 180.degree. phase difference from a second pair of other terminals, and with the four-way apparatus being utilizable for either a power-splitting function, i.e. wherein the common port receives input power and the four ports each provide approximately one-fourth of the input power as driving power to an associated input of a different power amplifier, or as a power combiner, i.e. wherein the output power received at each of the four other ports is combined into a signal at the common output port.
It is well known in the prior art to provide four-way splitter/combiner apparatus utilizing broadband transformers in which coaxial lines of severel different characteristic impedances, e.g. 25 and 50 ohm lines, are utilized with 4:1 impedance transformers, to provide four other port signals all having the same phase angle responsive to a single signal at the common port. Typical prior art configurations can be found in Application Note AN-749, entitled "Broadband Transformers and Power Combining Techniques for RF", in the Motorola RF Data Manual (1980) at pages 2-67 and 2-68.